1. Field of the Invention
The present invention is generally directed to the field of electrochemistry, and it relates to a new separation method and apparatus for removing ions, contaminants and impurities from water and other aqueous process streams. More specifically the invention relates to a new regeneration method for placing the removed ions back into solution.
2. Background Art
The separation of ions and impurities from electrolytes has heretofore been generally achieved using a variety of conventional processes including: ion exchange, reverse osmosis, electrodialysis, electrodeposition, or filtering. Several alternatives have been proposed that address the problems associated with the conventional separation processes. However, such alternatives have not been completely satisfactory for specific applications nor useful for all applications, and have not met with universal commercial success or complete acceptance.
The conventional ion exchange process has been used as a means for removing anions and cations, including heavy metals and radioisotopes, from process and waste water in various industries. This process generates large volumes of corrosive secondary wastes that must be treated for disposal through regeneration processes. Existing regeneration processes are typically carried out following the saturation of the columns by ions, by pumping regeneration solutions, such as concentrated acids, bases, or salt solutions through the columns. These routine maintenance measures produce significant secondary wastes, as well as periodic interruptions of the deionization process.
Secondary wastes resulting from the regeneration of the ion exchangers typically include used anion and cation exchange resins, as well as contaminated acids, bases and/or salt solutions. For example, H2SO4 solutions have been used for the regeneration of cation columns in metal finishing and power industries; HNO3 solutions have been used for the regeneration of cation columns used in processing nuclear materials; and NaCl solutions have been used in residential water softening processes.
Under an applied electrical field, ionic species in solution can be separated from the aqueous solution by a variety of means. The use of flow-through or flow-by electrochemical cells containing porous, high-surface-area, electrically conductive carbonaceous electrodes have been employed for separation of ionic species in solution. The electrodeposition of metals from aqueous solutions where electron transfer takes place between the carbon electrode and the ions in solution has been employed. Without apparent electron-transfer steps at potentials less than the reduction potentials of the soluble species, ionic species are thought to be separated from the solution by a simple electrostatic separation where they are held within the electrical double-layer formed at the solution-electrode interface, i.e., capacitive deionization. Such deionization of the solution by a capacitive process has been proposed in the early 1960""s by Arnold and his colleagues [B. B. Arnold, G. W. Murphy, J. Phys. Chem. 65 1 (1961) 135-138.]. Their capacitive deionization (CDI) process was suggested for desalination of brackish water.
In one capacitive deionization method described in U.S. Pat. No. 5,425,858 issued to Farmer (herein referred as xe2x80x9cFarmerxe2x80x9d), a stream of electrolyte to be processed, which contains various anions and cations, electric dipoles, and/or suspended particles, is passed through a stack of electrochemical capacitive deionization cells, i.e., a capacitor. Each of these cells of the capacitor includes numerous electrodes having exceptionally high specific surface areas (for example, carbon aerogel having a surface area of 400-1000 m2/gm). By polarizing the cell, non-reductible and non-oxidizable ions are removed by electrodeposition. Electric dipoles also migrate to and are trapped at the electrodes. Small suspended particles are removed by electrophoresis. Therefore, the fluid stream leaving the cell is purified.
The Farmer method is an efficient deionization process since the pressure drop in the capacitive deionization cell is dictated by channel flow between parallel surfaces of monolithic, microporous solids (i.e., the electrodes) and across (rather than through) such surfaces. Hence, it is insignificant compared to that needed to force water through the permeable membrane required by the reverse osmosis process as well as capacitive deionization processes described by Andelman in, for example, U.S. Pat. Nos. 5,547,581, 5,415,768, 5,360,540, 5,200,068, and 5,192,432.
A feature of the Farmer separation system is that no expensive ion exchange membranes are required for the separation of the electrodes. All the anodes and cathodes of the electrode pairs define individual cells that are connected in series, each pair defining an open, unobstructed channel for fluid flow between the electrode pair. The system is modular and can be readily expanded to include several electrode pairs (i.e. cells or modules) thus forming a capacitor with a relatively large anode or cathode total surface area. Typically, the electrode pair modules are arranged so that fluid flow through the capacitor is in a serpentine pattern across, rather than through, a relatively large number of intermediate electrode pairs having no dimension open to the exterior of the cell(s) or capacitor (other than those electrode pairs at a single fluid input and single fluid output location). Ultimately, the Farmer system capacity can be increased to any desired level by expanding the capacitor to include a substantial number of electrode pairs. Although each electrode pair can define a single cell, in the Farmer system all of the closed series of cells formed by the intermediate electrode pairs act as a single cell or capacitor.
The Farmer system is not without its problems; underpotential deposition, electrodeposition, chemisorption, electrophoresis and other separation phenomena involving a charge transfer across the carbon surface and the liquid electrolyte can also occur. These types of processes affect the reversibility and robustness of the deionization-regeneration cycle (process). Under such conditions, the regeneration and/or rejuvenation of the saturated electrode and cell system would dictate the performance and overall effectiveness of the deionization process. Recent efforts have focused on the efficiency and effectiveness of the regeneration of multiple-cell stacks containing the above-described carbon aerogel compounds.
The electrosorption of simple ionic compounds such as NaCl, NH4Cl and NaNO3 may be accomplished reversibly if cells can be regenerated or rejuvenated with deionized water over a long period of time and with voltage reversal as appropriate. However, several techniques for effective and optimal regeneration methods have been suggested. Even under a low or mild applied potential (i.e., less than 1.2V in water-based streams) for removal of ions, certain charge transfer processes have been shown to occur which have given rise to deposited or strongly bound species. During regeneration, even with a reversed polarity, the applied voltage to capacitive systems may not be sufficient to conteract fractions of the ions that remain attracted strongly by other means such as underpotential deposition or chemisorption. Accordingly, a cycle efficiency (i.e., the ratio of the amount of salt recovered during regeneration over that determined during deionization expressed in terms of percentage) never reaches 100%, particularly over many successive cycles and especially when the total regeneration time and the total deionization time is the same. Certain features of such inherent inefficiency have been observed in results presented in previous investigations. Under special treatments, aged electrodes can, however, be rejuvenated to regain degraded capacity. Farmer et al. in the Journal of the Electrochemical Society, 143 pp. 159-169, 1996) has reported that electrodes that have been aged naturally for many experiments over several months have been rejuvenated to close to the original capacity by reversing the polarity for a short period of time. U.S. Pat. No. 5,620,597, issued to Andelman, has suggested that electrodes of carbon fibers and activated carbon powders (arranged to form short fluid pathways, each open in at least one dimension to the exterior of the capacitor) can be maintained by alternating the polarity in successive cycles. However, limited data have been collected on the regeneration of such saturated electrodes on an electrochemical cell basis. Presently, no process has effectively shown the efficacy and efficiency of continuously processed deionization-regeneration cycles.
Regeneration of the Farmer system at the same or lesser time than the actual deionization step in a given cycle has not been shown. Furthermore, contaminated water usually contains a variety of ions and species with different properties. Use of chemical regenerants, deionized water, long regenerating time and voltage reversal is needed to completely rejuvenate the system if such a system is not regenerated immediately and allowed to age under such extreme conditions.
Timely regeneration and activation of electrodes can allow brief extension of the operating voltage window to thus expand the capacity of the electrodes without irreversibly damaging the electrodes. Accordingly, deionization (e.g., desalination) can be accomplished at potentials above that of water electrolysis which is 1.2V. A short voltage pulse above 1.2 V can allow more efficient enhancement of the capacity without generating significant gas amounts by electrolysis. Subsequent operation at a reversed voltage will rereact this small amount of gases and reversibly restore the electrode capacity.
Thus, a need exists for a new method/technique for a complete deionization-regeneration cycle which continuously maintains the efficacy and efficiency of the deionization system. Such a process can significantly reduce, if not entirely eliminate, secondary wastes in certain applications, and does not cause a considerable pressure drop in the flowing process stream or require significant energy expenditure. Deionization methods should not require a short fluid pathway in electrosorption systems, should not require salt additions for ion regeneration in a water softening system, and further should not require additional desalination devices, such as reverse osmosis filters, to remove the excess sodium chloride introduced during regeneration. Additionally, the new method should enable the separation of any inorganic or organic ion or dipole from any ionically conducting solvent, which could be water, an organic solvent, or an inorganic solvent. For example, it should be possible to use such a process to purify organic solvents, such as propylene carbonate, for use in lithium batteries and other energy storage devices. Furthermore, it should be possible to use such a process to remove organic ions, such as formate or acetate from aqueous streams.
The new method should further be adaptable for use in various applications, including without limitation, treatment of boiler water in nuclear and fossil power plants, production of high-purity water for semiconductor processing, removal of toxic and hazardous ions from water for agricultural irrigation, and desalination of sea water.
Therefore, there is still a significant unfulfilled need for a new method and apparatus for deionization and regeneration which, in addition to the ability to significantly reduce if not to completely eliminate secondary wastes associated with the regeneration of ion exchange columns, do not result in a considerable pressure drop of the flowing process stream, and do not require significant energy expenditure. Furthermore, each electrode used in an apparatus employed for deionization and regeneration should be made of a structurally stable, porous, monolithic solid. Such monolithic electrodes should not become readily entrained in, or depleted by the stream of fluid to be processed, and should not degrade rapidly with cycling. These electrodes should have a very high specific surface area; they should be relatively thin, require minimal operation energy, and have a high removal efficiency. The new method should be highly efficient and should be adaptable for use in a variety of applications including, but not limited to, sea water desalination.
A separation method or system, referred to as capacitive deionization (CDI), is employed for the deionization of water containing electrolytes and the treatment of aqueous wastes. Such a system is designed so that the deionization can be accomplished by an applied electrical potential, preferably reversiblyxe2x80x94even where the applied voltage exceeds about 1.2 V up to about 1.7 V. The inventive process involves a combination of deionization and regeneration steps in each cycle under conditions whereby the capacity to remove charged contaminant particles is greatly enhanced and the system can be reversibly regenerated over the course of several cycles, preferably during alternating cycles. Unlike conventional ion exchange processes, no chemicals, whether acids, bases, or salt solutions, are required for the regeneration of the system; instead, electricity is used.
A fluid stream of electrolyte to be treated, which contains various anions and cations, electric dipoles, and/or suspended particles, is passed in a serpentine path through a stack of electrochemical capacitive deionization cells (i.e., a battery of connected, spaced apart, anode/cathode pairs, each pair operable as an electrochemical cell). Each of these cells contains high surface area, low resistive, e.g., less than electrodes, preferably including carbon aerogel electrodes having exceptionally high specific surface areas (for example, 400-1000 m2/gm). By polarizing the cell, non-reducible and non-oxidizable ions are removed from the fluid stream electrostatically and held in the electric double layers formed at the surfaces of the electrodes and the fluid stream leaving the cell is purified. Some metal cations are removed by electrodeposition. Electric dipoles also migrate to and are trapped at the electrodes. Small suspended particles are removed by electrophoresis.
In a preferred embodiment, the contaminated fluid is passed through an open channel defined between at least two pairs of intermediate electrodes of the battery of electrochemical cells operating at a positive polarity so as to deionize the fluid. The open channel between the intermediated electrodes has no dimension open to the exterior of the battery. The deionization step is interrupted to electrostatically regenerate the battery, and an additional portion of the contaminated fluid is then passed through the battery operating at a negative polarity to deionize an additional portion of the fluid.
In the present CDI process, energy is expended using electrostatics to remove salt and other impurities from the fluid, and, as a result, is more energy efficient than previous processes. Furthermore, the pressure drop in the capacitive deionization cells is dictated by open channel flow between generally parallel surfaces of monolithic, microporous solids (i.e., the electrodes); hence, such pressure drop is insignificant (i.e., less than about 5 psig) compared to that needed to force water through the permeable membrane required by the reverse osmosis process.
More particularly, the invention relates to the regeneration of the saturated or nearly saturated cells that significantly affect the overall purification/regeneration cycle and its efficiency in continuous operation. The most effective means for achieving and maintaining high capacity and regeneration efficiency involves systematically alternating the polarity of the stack in successive deionization (i.e., purification) steps. The multiple-cell stack (i.e., battery) operating under this condition and with a serpentine fluid (and electrolyte) flow path, retains more than 95%, and in some cases, close to 100% of its capacity after many cycles. A series of experiments employing the multi-cell serpentine flow stacks of the invention has exhibited a product recovery ratio exceeding that for a comparable multi-cell stack having parallel open channel anode/cathode pair cells with at least one dimension open to the exterior.
The CDI separation system has no expensive ion exchange membranes required for the separation of the electrodes. All the anodes and cathodes of the electrode pairs are electrically connected in parallel and the stacked multi-cell capacitor is arranged so the unobstructed, open channel defined by the anode and cathode pairs allows the serpentine path for the fluid stream. The capacity of the modular inventive system can be increased to any desired level by expanding the cell(s) to include at least three, at least four, and preferably several electrode pairs in the battery. Although at least one pair of anode/cathode electrodes at fluid input and output from the battery has a dimension open to the exterior of the capacitor, the majority, and normally greater than 90%, of the intermediate electrode pairs arranged in the interior of the stack have no dimension open to the exterior.
Advantages of the present invention include, but are not limited to, the following:
1. In the present system, electrolyte flows in open channels formed between two adjacent, planar electrodes, which are usually geometrically parallel, although nonparallel configurations apparent to those of ordinary skill in the art may also be employed. Consequently, the pressure drop is much lower than conventional processes. The fluid flow can be gravity fed through these open channels, or a pump can be used.
2. The CDI system does not require membranes, which are both troublesome and expensive, which rupture if overpressured, which add to the internal resistance of the overall capacitive cell (capacitor), and which further reduce the system energy efficiency.
3. The electrodes in the CDI system encompass immobilized sorption media, such as monolithic carbon aerogel, which is not subject to entrainment in the flowing fluid stream. Thus, material degradation due to entrainment and erosion is considerably less than in conventional packed carbon columns.
4. The present systems and methods are inherently and greatly energy efficient. For instance, in both evaporation and reverse osmosis processes, water is removed from salt, while in the present systems, salt is removed from water, thus expending less energy.
5. The present systems and methods present superior potential distribution in the thin sheets of carbon aerogel; most of the carbon aerogel is maintained at a potential where electrosorption is very efficient, such as around about 1.2 v.
The above advantages of the present invention are realized by new electrically regeneratable electrochemical cells (battery) for capacitive deionization and electrochemical purification and regeneration of electrodes. The cell includes two end plates, one at each end of the cell, as well as two end electrodes that are arranged one at each end of the cell, adjacent to the end plates. An insulator layer is interposed between each end plate and the adjacent end electrode.
Each end electrode includes an electrosorption medium having a high specific surface area and sorption capacity. In the preferred embodiment, the electrosorption medium is formed of carbon aerogel composite. The battery further includes one or more intermediate electrodes that are disposed between the two end electrodes. As the electrolyte enters each cell, it flows through a continuous open serpentine channel defined by the electrodes, substantially parallel to the surfaces of the electrodes. By polarizing each cell, ions are removed from the electrolyte and are held in the electric double layers formed at the carbon aerogel surfaces of the electrodes. As each cell is saturated with the removed ions, each cell is regenerated electrically, thus significantly minimizing secondary wastes.